A Practical consideration
of products, methods,
North American Cement Corporation
HAGERSTOWN - - - MARYLAND
TABLE OF CONTENTS
Introduction ......"......... 3
Fundamental requirements for good concrete work :
Acceleration: Possible — Practical — Economical 5
Effect of accelerators on the strength of concrete 9
Effect of curing on the strength of concrete 11
Effect of water ratio on the strength of concrete 12
Reducing the water through the use of Cal . 14
Aids in frost-proofing and in densifying 15
Use of Cal to correct unsoundness in concrete 16
Comparative costs of concrete accelerators 18
Calcium chloride in disguised forms 20
THE constant development of new products and new
methods in concreting, the constantly broadening use
of Portland cement in every type of construction, the
increasing variety of special conditions which it is called
upon to meet, and the growing pressure for speed, have led
us to prepare this brief consideration of the most economical
and effective methods of handling concrete work in which
early set and early strength are of importance.
In recommending the use of our particular product, Cal,
to meet certain requirements of modern concreting, we have
carefully limited ourselves to those uses which the tests of
experts and the experience of thoroughly capable and prac-
tical men have amply demonstrated.
The properties of Cal which are here generally dis-
cussed may readily be applied by the reader to his individual
problems of road and street building, reinforced concrete
construction, stucco, brick mortar,* concrete products.
Our technical staff will be very glad to consult with any-
one interested in any phase of the concreting problems dis-
cussed in this booklet.
*The effect of Cal on mortar for use in brick const)~uction, is in
general, the same as outlined in the following pages for concrete. It
is important to note, in this connection, however, that five years of
job experience have confirmed the finding of the U. S. Bureau of
Standards (see Technological Paper No. 174, page 12) that the ad-
dition of Cal to brick mortar does not cause efflorescence.
Fundamental Requirements for Good Concrete Work
N the making of the best Concrete and Mortar, there is
no substitute for the selection of proper materials and
insistence upon good workmanship.*
1. Clean sand and stone.
2. Well graded aggregates.
3. Mixing water that is fit to drink
1. The least amount of gauging water should be used
that will give the workability necessary for the job. Keep
reducing the amount of water until you find the point where
the mixture is too stiff to work, and then add a little bit
extra, (a pint often makes the difference in a 1-bag batch)
until it can be handled easily.
2. Long mixing. Mix concrete for at least one minute
and mortar for at least two minutes.
*Valuable facts have been gathered by the Portland Cement Asso-
ciation on these points, and are fully given in its pamphlet entitled
"Concrete Data for Engineers and Architects." Copies can be ob-
tained free by application to any one of the numerous district offices
of the Association, or to the headquarters at Chicago, III
1. Protection from freezing. (See pp. 15-16).
2. Protection from hot sun or drying winds. Just as
soon as the concrete or mortar is placed, protect the exposed
surfaces from the warm sun and from wind and other air
currents, wherever job conditions make this possible, and
continue the protection for several davs.
Acceleration : Possible — Practical — Economical
THE modern need for accelerating the hardening of
Portland Cement mixtures is well presented in the
following statement from Technologic Paper No. 174
of the U. S. Bureau of Standards :*
"A demand for a practical and efficient material for ac-
celerating the hardening of Portland cement mixtures has
received considerable attention in recent years. The need
for such a material becomes of more consequence with the
introduction of more rapid methods in construction and with
the increasing amount of concrete repair and replacement
work. Serious problems arise when it becomes necessary to
interrupt or divert traffic for the duration of time required
for concrete to harden sufficiently. The increased cost of
lumber for forms has necessitated a more judicious use of
such material, and its removal for further use as soon as
the strength of the concrete will allow. This applies even
more to the use of steel forms, which are rapidly replacing
wooden forms in a great many types of concrete construc-
~i 5 h
"Considerable work toward the development of an ac-
celerator for concrete has been done by the U. S. Bureau of
Standards, which found that calcium chloride was the most
effective of the various substances tested."
The rate of acceleration to be obtained with calcium
chloride is well know, and is generally accepted as adequate
in all jobs except those which absolutely require a highly
exceptional rate. However, as the paper above quoted goes
on to say : " . . . . the use of commercial calcium chloride
in concrete is attended by difficulties caused by its highly
hj^groscopic property and by the handling of the solution."
One of these difficulties is that calcium chloride is added
in proportion to the volume of gauging water, which may
vary from 25 to 45 gallons per cubic yard of a 1:2:4 mix,
according to the dryness or wetness of the aggregates used,
and without any relation to the amount of cement which
the calcium chloride is intended to harden. The proportion
is, therefore, extremely variable and the results necessarily
uncertain. Too low a percentage of calcium chloride reduces
the rate of hardening — too high a percentage may give
'"flash set" and seriously affect the ultimate strength of the
The second difficulty in using free calcium chloride is
that, when it has been added to a barrel of gauging water.
it disappears and leaves no trace which the man at the mixer
an recognize. This invites the danger of a double dose, or
no dose at all.
The third difficulty is that free calcium chloride is highly-
electrolytic and is, therefore, not advisable for use between
or near electric railway tracks — where it may corrode the
rails and also develop checks and cracks in the concrete
from the electrolytic action.
Chiefly as a means of making acceleration a safe, prac-
tical and economical operation under the conditions of aver-
age construction work, and with the grade of labor ordin-
arily employed, our chemical staff developed a compound
which uses calcium chloride as a base, yet eliminates to a
large degree the amount of labor and the chances of error
involved in mixing.
This compound, Cal, is a dry white powder which re-
quires no solutions or extra preparations, but is added di-
rectly to the cement and aggregates, at the mixer.
In the form of Cal, the corrosive properties of calcium
chloride are entirely neutralized. Cal alone is actually
an inhibitor of corrosion.
Regarding the accelerative value of Cal, we quote again
from U. S. Bureau of Standards Technological Paper No.
174 (page 7) :
"The effect of Cal on the setting time of normal Port-
land cements may be seen by examing the results given
below. The set was hastened in all cases and to a greater
extent by the larger percentage of Cal."
Cal to cement
From Technologic Paper 171. V. S. Bureau o1 Standards.
The official conclusion drawn from the tests here tabul-
ated, are given on page 22 of the above paper.
"The setting of normal Portland cement mixtures may be
hastened by Cal to an extent which is very desirable in
concrete construction requiring a finished surface. The
finishing operation may proceed with much less delay after
the concrete has been placed, which should result in cut-
ting down overtime labor. This hastening of the set is
i 8 y
not objectionable in any type of construction providing the
concrete is placed soon after it is gauged with water.
"Cal was used in a concrete floor topping alongside of
topping containing no accelerator, for the purpose of ob-
serving the effect under actual field conditions. The treated
topping was ready for the finishing operation in about one-
half the time required by the untreated, as judged by the
finisher. This agreed very closely with the results of the
laboratory tests on the rate of setting of the two mixes by
means of the flow table."
If the calcium chloride could be eliminated from Cal
without reducing the effect on fatness and workability, and
if full advantage were taken of the drier mix which this
workability allows, a very considerable degree of accelera-
tion would still be achieved.
In the case of Cal, there is actually a double accelera-
1. That produced by its calcium chloride content.
2. That produced by the lower water ratio which its
fattening effect makes possible.
Effect of Accelerators on the Strength of Concrete
WHETHER calcium chloride alone, or calcium chloride
in the form of the Cal compound, be used, it has
been established that a rapid increase of strength
may be looked for.
The conclusions with regard to the operation of Cal
are summarized in the following quotation from pages 22
and 24 of the U. S. Bureau of Standards Technologic Paper
~i 9 h
"All Portland cement mixtures treated with Cal at-
tained greater strength at the 2-day and 7-day periods than
the corresponding untreated mixtures. The percentage in-
crease in the strength of mortar at the 2-day period ob-
tained by an addition of 5 per cent Cal to cement ranged
from 40 to 140. The strength of the treated mortar at two
days was equal to the strength of the untreated mortar at
three and one-half to eight days. These calculations are
made from the results of tests in which the test pieces were
stored in water, damp sand, or damp closet.
"Treated mortars stored in the laboratory air attained
at 2 days strength greater than that of the untreated mor-
tars at 28 days. This was due to the rapid drying out of
the small test pieces and the comparatively slow rate of
gain in strength after the 2-day period. However, this
indicates that Cal is especially advantageous in cement
mixtures which are necessarily subjected to any drying-out
"The increase in strength produced by 5 per cent Cal
in concrete mixtures at the 2-day period ranged from 52
to 135 per cent, and the strength of the treated concrete
at the 2-day period was equal to that of the untreated at
from three to four and one-half days. On an average this
represents a saving of approximately one-half the time in
operations which are dependent upon the strength of the
concrete at early periods. The effect of the air storage in
the concrete tests was lessened in degree, owing to the high
relative humidity which existed throughout the storage
"It should be remembered that the increase in strength
resulting from an addition of 5 per cent Cal does not repre-
sent the maximum which may be obtained except in very
-4 i o y-
rich mixes. As much as 15 per
tar tests, giving an increase of
cent Cal was used in mor-
220 per cent at the 2-day
Effect of Curing on the Strength of Concrete
THE importance of controlling the conditions of curing
concrete after it has been placed should not be over-
looked. It has been well established that a moist at-
mosphere at 70° F. presents the ideal curing condition for
practically all types of concrete construction, and that the
maintenance of this condition gives maximum strength, du-
rability and density to the finished structure. Such ideal
curing conditions, of course, can rarely be obtained on the
actual job. Variations in temperature and in humidity may
extend over wide ranges, and unless special precautions are
used, the resulting concrete will be equally variable.
The most severe condition which can be met in ordinary
construction work during the curing period is produced by
very hot and very dry weather. In such weather concrete
is dried out rapidly from the surface, so that there is not
enough moisture left to properly combine with the cement.
The result is a soft-cured, porous and cracked surface,
which offers easy entrance to rain, frost and other destruc-
tive agents of the atmosphere.
Where Cal is used with concrete cured under such dry-
ing-out conditions as are described above, the results are
particularly interesting and encouraging. They are sum-
marized as follows in the U. S. Bureau of Standards Tech-
nological Paper No. 174 Page 10) :
"The effect of Cal vnder the conditions of these tests is re-
markable. The strength of the treated mortar at S days is con-
siderably above that of the untreated mortar at 28 days."
H[ 1 1 t-
The action of Cal under these drying-out conditions
seems to be two-fold :
It accelerates the set and strengthening of the con-
crete or mortar so that the period in which serious
drying-out is possible is reduced to the minimum.
2. Cal has the property of retaining the water of the
mix and resisting evaporation.
For all work in which surfaces are necessarily exposed
to sun and wind, this property of Cal will be found of real
Effect of Water Ratio on the Strength of Concrete
THROUGH the long and careful investigations of the
Portland Cement Association, it has been demon-
strated that the strength and durability of a finished
concrete job depends in a very considerable degree upon the
percentage of water used in the original mix. The official
report of this Association makes the following statements:
"Excess mixing water weakens concrete. Sloppy mix-
tures sacrifice strength. One pint more water than neces-
sary in a one-bag batch decreases the strength and resist-
ance to wear of concrete as much as if two or three pounds
of cement were left out. Concrete hardens because of chem-
ical reactions between Portland cement and water. The
quantity of mixing water is just as important as the quan-
tity of cement.
-Proper £€>n*jftrncy for mas* ca/itrrrfe,
concrete A/itft>*&Y0ar t f/T?*'/7tj M efc
r h/s rat??* &fc&rts/ste/icy sAwfe/
Ar used for cast pnxf acts; r€/r?fbnceo
cpncrcfe, stcjt&f/? members raqu/ra
\/fte frt&fer amount &f prsfrr
•With this consistency about"
one-iha/f the strength h~ tost
With the sfoppy "concrete some -
'•nes used in rood work and /n
but. 'ding construct/on, t#o- thirds
to three-fourths of the possio/e
strength of the concrete is hst
ISO (60 ITO /SO (30 tOO
fO iO *0 tOO HO fZO 130 14-0 t&v ivv rtv wv rx too
Wtter Used- Figures are percent- of Cuantity Giving "Maximum ftrength^
Fig. l. Effect of Quantity o£ Mixing: Water on the Compressive Strength
of Concrete. Note: In general construction, the maximum strength can
rarely he obtained, but it is possible to obtain 70 to 90 per cent of the
maximum strength without additional expense by restricting the quantity
of mixing water.
With given proportions, the
should obviously be reduced as
tain a plastic mixture.
"The strength of
concrete depends on
the ratio between
the volume of mix-
ing water and the
volume of cement
( J ). As long as
the mixture is work-
able, the smaller the
the stronger the
concrete. Thus de-
creasing the quan-
tity of mixing water
and increasing the
quantity of cement
both add to the
strength of concrete,
quantity of mixing water
far as possible and still ob-
"The curve in Fig. 1 shows the effect of the quantity of
mixing water on the strength of concrete. It is an average
of the results obtained with many mixtures and aggregates.
The data on which it is based were secured from several
thousand tests at the Structural Materials Research Labor-
atory, Lewis Institute, Chicago, 111. Mixtures on the right
of the maximum strength line were plastic; mixtures on
the left were not.
"In general construction work, maximum strength can
seldom be secured, because the mixture would be too stiff
to be workable, but 70 to 90 per cent of the maximum
strength can readily be obtained. This would be a great
increase over the usual results, since much of the concrete
placed today contains 50 to 100 per cent more water than
necessary and thus attains only half or even only a quar-
ter of its possible strength.
"Resistance to wear, which is vital in concrete pave-
ments, floors and sidewalks, increases with compressive
strength. Thus, the smaller the quantity of mixing water
the more wear-resistant will be the concrete."
Reducing the Water Ratio Through the Use of Cal
As an aid to reducing the water content, Cal will be
found efficient. One of its ingredients is a specially precipi-
tated hydrated lime which acts as a lubricant to the mix,
making it "fatter" and more easily workable — so that a mix
with less than the usual amount of water will flow quite as
readily and be as easily handled on the shovel or trowel, or
through a chute. The additional fatness and workability
given by Cal produce a mix that reaches the bottom of
the chute in the same uniform condition, with the same
ratio of cement and aggregates throughout that it had
when it left the top.
In this connection, it is important to note that Pearson
and Hitchcock, after making an exhaustive test of worka-
bility came to the conclusion that hydrated lime in the form
in which it is present in Cal is superior to ordinary dry
hydrated lime in effecting workability of the mix and thus
reducing the water ratio required.
■i u h
Their conclusions are expressed as follows on Page 293
of the Technical Papers (Part II, Vol. 23, 1923) of the
American Society for Testing Materials:
"Accelerator 'A'* is a calcium oxychloride which quickly de-
composes to calcium chloride and hydrated lime when mixed
with water. The hydrated lime thus freed is much more effec-
tive upon workability, as indicated by the results of these tests,
than the dry hydrated lime."
Reference again to the U. S. Bureau of Standards table
quoted on Page 8, shows that with an 8% addition of
Cal the water content for usual flow is about 2*4% less
than required for mixes to which no Cal has been added.
Aids in Frostproofing and in Densifying
FOR lowering the freezing point of the mix, and hasten-
ing its set so that it is exposed for a shorter period to
the possibility of freezing, Cal will be found a valu-
able and dependable aid.
In general, Cal will render mortar or concrete proof
against freezing at 10 degrees below the temperature at
which the untreated mix could safely be used.
_ When concrete is placed at low temperatures, the most
critical period of setting is during the first few hours im-
mediately after the concrete is poured. After the setting
action is well started, there is far less likelihood of damage.
Cal is particularly effective at the earliest stages after the
concrete is poured in hastening- the setting- action of the
cement. Also, because of its fatness and the consequent
absence of "free water" in properly mixed Cal-Concrete,
the possibility of freezing is still further reduced.
The use of Cal should not, of course, cause neglect of
the well-recognized precautions for concreting in extreme
weather, such as heating aggregates, heating water, cover-
ing finished work, etc.
The water-proofing characteristics of Cal are chiefly
the result of the specially precipitated hydrated lime which
forms an important part of the compound, and which in
a less effective form has long been used independently fo
this purpose. The fatter mix obtained with Cal insures
more compact and more complete filling of molds and forms,
and closes up the pores of the concrete with verj r fine par-
ticles of hydrated lime. Its chemical action insures a thor-
ough cure and the complete hydration of the cement through
and through which is necessary for perfect and lasting
Use of Cal to Correct Unsoundness in Cement
IN cases where the steam test shows an inferior quality
of cement and where circumstances make it impossible,
difficult or unduly costly to wait for a fresh shipment,
the unsoundness can be corrected by the use of Cal. This
has been amply demonstrated in actual field operations, and
is authoritatively expressed as follows by the U. S. Bureau
of Standards in Technologic Paper No. 174 (Page 9) :
"A very unsound cement was used in a 1:3 mortar which
was molded into 2-inch cubes and stored in air after 24
hours. At the end of 8 months, the plain mortar test pieces
were entirely disintegrated. The test pieces made from the
same mortar to which 5 per cent of undried Cal was added
were still very firm, although a few disintegration cracks
Soundness after addition of various percentages of undr ied Cal
_ per cent
5 per cent
8 per cent
. . .do
ram Technologic Paper 17 A, U. S. Bureau of Standards.
The conclusion from these tests is expressed on page 22
of the above paper in these terms: "Unsound cements may
be greatly benefitted or made sound by an addition of Cal."
Comparative Costs of Concrete Accelerators
IF one considers only the cost of the actual product re-
quired to produce a given degree of acceleration, com-
mercial calcium chloride is, of course, cheapest. This
chemical may be obtained in the granular or flake form, and
is made into water solution before use. Its cost, when care-
fully used, is about 17 cents per barrel of cement treated,
as compared with a cost of 60 cents for Cal, costs of $1.00
to $4.00 for the various "patent medicine" forms of Calcium
Chloride, and about $4.50 per barrel more than Portland
Cement for special quick hardening cements of the high
For work where acceleration only is sought, and where
the size and stationary character of the job permits the
establishment of adequate equipment for the convenient
storage and handling and the accurate proportioning and
testing of the accelerator, calcium chloride will give satis-
factory results at minimum cost.
In a large number of cases, however, the use of Cal, at
the increased cost, will be justified. This is especially true
where the concrete mixer is in motion (as in road and street
work), where water connections have to be changed re-
peatedly, and where there can be no permanent provision
for the preparation, storage, and analysis of the accelera-
tor. On such jobs, the saving of time and the assurance of
accurate proportioning alone warrant the extra cost.
A 1 8 *
In addition, however, it must be borne in mind that for
the actual amount of calcium chloride contained in Cal,
a higher degree of acceleration can be obtained because of
the drier mix which Cal permits and the increased setting
speed produced by this absence of the "free water" which
And, finally, where the ultimate durability of the con-
crete is given proper consideration, the increased fatness
and improved cure of a mix obtained with Cal, will insure
a permanence and quality which makes its increased cost a
comparatively small item.
For the use of the "patent medicine" compounds dis-
cussed on the next page there is no real justification in re-
i Us for the high costs fixed on these products.
In those exceptional cases where very high strengths
are needed in 24 hours, it will often be found advisable to
use special cements, but it is always necessary to compare
costs with the actual saving in time which can be realized.
Frequently it will be found that Cal and Portland cement
will give all the strength necessary for the particular job
in very little more time than the special cements.
Calcium Chloride in Disguised Forms
IT is a matter of common knowledge that herbs and :hem-
icals of proved medicinal value and comparatively low
cost are largely sold in compounds bearing a trade name
at a tremendous increase in price.
The same thing- has happened in the case of calcium
chloride. Its acknowledged property of acceleration, and the
protection this gives against freezing, has caused it to be
used as the principal ingredient of a great variety of trade-
marked products sold under names suggestive of these de-
sirable accelerating and frost-proofing results.
Practically all these "patent medicine" products have one
merit — the merit that results from the net amount of actual
calcium chloride which they contain. But it will be found,
upon chemical analysis, that this merit is the only one
which they possess — and that other ingredients are added
merely to disguise the base and to make plausible the tre-
mendous increase in price over the cost of the calcium
chloride. This increase runs anvwhere from 300% to
While Cal is also a compound with a base of Calcium
Chloride its exact constituents have been made a matter of
public record through the analysis of the U. S. Bureau of
Standards, given in Technologic Paper No. 174 and its added
advantage of convenience, safety, labor-saving, double ac-
celeration, and improved durability in the resulting concrete
give ample warrant for its higher cost as compared with
ordinary calcium chloride.
bj*PA«TK*KT Of COMMIRCI
Bureau of Standards
6. W. 8TRATTOW. Di»»cto»
EFFECTS OP CAL AS AH ACCELERATOR
OF THE HARDENING OF PORTLAND